1. Field of the Invention
This invention relates to frame structures for communication systems and more particularly to frame structures for adaptive modulation wireless communication systems.
2. Description of Related Art
A wireless communication system facilitates two-way communication between a plurality of subscriber units (fixed and portable) and a fixed network infrastructure. Exemplary communication systems include mobile cellular telephone systems, personal communication systems (PCS), and cordless telephones. The key objective of these wireless communication systems is to provide communication channels on demand between the plurality of consumer subscriber units and their respective base stations in order to connect the subscriber unit user with the fixed network infrastructure.
Subscriber units typically communicate through a terminal with the base station using a “duplexing” scheme thus allowing the exchange of information in both directions of connection. Transmissions from the base station to the terminals are commonly referred to as “downlink” transmissions. Transmissions from the terminals to the base station are commonly referred to as “uplink” transmissions. In wireless systems having multiple access schemes a time “frame” is used as the basic information transmission unit.
Depending upon the design criteria of a given system, systems have typically used either time division duplexing (TDD) or frequency division duplexing (FDD) methods to facilitate the exchange of information between the base station and the terminals. In a TDD communication system, the base station and the terminals use the same channel, however, their downlink and uplink transmissions alternate one after the other to prevent interference. In a FDD communication system, the base station and the terminals use different channels for their downlink and uplink transmissions, respectively. Thus, the concern for interference between uplink and downlink transmissions is mitigated in a FDD communication system as compared to a system using TDD. However, the increased cost and complexity in deploying a FDD communication system often outweighs this obvious advantage over a TDD communication system.
In both TDD and FDD systems, each base station and terminal includes a modem configured to modulate an outgoing signal and demodulate an incoming signal. If the modem is configured to modulate and demodulate simultaneously, the modem is a “full-duplex” modem. If the modem is not configured to modulate and demodulate simultaneously, but rather switches between modulating and demodulating, the modem is a “half-duplex” modem.
In an exemplary FDD communication system, each terminal's modem operates simultaneously to transmit and receive information in a full-duplex manner. Such a terminal can continually receive data from the base station. By continually receiving information, the terminal is able to maintain its synchronization with the base station. By maintaining its synchronization, the terminal is less dependent on the base station transmitting control information and preambles to assist the terminal in locating its data within the downlink.
Because a half-duplex terminal does not receive information from the base station when the terminal transmits it uplink to the base station, it may fall out of synchronization with the base station. When this occurs, the terminal may require the base station to downlink additional control information or a preamble to allow the terminal to re-synchronize prior to it receiving downlink data from the base station.